1. Field of the Invention
This invention relates to a display device used for a display unit such as of a portable electronic equipment, a light source device used therefor, and a light guide plate.
2. Description of the Related Art
Accompanying the widespread use of portable data terminals, it has been urged to provide a display device which consumes small amount of electric power. The reflection type liquid crystal display device realizes a low power consumption accompanied, however, by such a problem that a good display quality is not obtained unless the display device is used in a sufficiently bright environment such as outdoors. Therefore, the reflection type liquid crystal display device is generally used in combination with a front-light unit which is arranged on the side of the display screen and is illuminated with a plane source of light.
As a source of light for the front-light unit, there is used a cold cathode tube or a light-emitting diode (LED). An LED which is light in weight and is small size has been frequently used for relatively small liquid crystal display devices. Unlike the cold cathode tube which is a linear source of light, the LED is a point source of light. To uniformly illuminate the display screen, therefore, a structure is necessary for uniformly spreading light.
FIG. 22 is a view illustrating the constitution of a conventional front-light reflection type liquid crystal display device. FIG. 22A is a view illustrating the constitution of the reflection-type liquid crystal display device as viewed from the side of the display screen, and FIG. 22B is a sectional view of the reflection-type liquid crystal display device cut along the line A—A in FIG. 22A. Referring to FIGS. 22A and 22B, the reflection-type liquid crystal display device is constituted by a front-light unit FL and a reflection-type liquid crystal display panel 102. A polarizing plate 140 is stuck to the surface of the reflection-type liquid crystal display panel 102 on the side of the display screen. Further, a transparent planar light guide plate 104 that constitutes a portion of the front-light unit FL is arranged on the side of the display screen of the polarizing plate 140 maintaining a predetermined gap. A nearly flat light outgoing surface 128 is formed on the planar light guide plate 104 on the side of the liquid crystal display panel 102. A light incidence surface 122 on which light emitted from the source of light will fall, is formed on the left side of the planar light guide plate 104 in FIGS. 22A and 22B. On the planar light guide plate 104 on the side of the display screen, there are alternately formed a plurality of mildly inclined surfaces 112 that are inclined at a relatively small angle toward the light incidence surface 122 with respect to the light outgoing surface 128 and a plurality of steeply inclined surfaces 110 that are inclined at a relatively large angle toward the surface 144 facing the light incidence surface 122 with respect to the light outgoing surface 128.
On the left of the planar light guide plate 104 in FIGS. 22A and 22B, a linear light guide plate 106 is arranged along the light incidence surface 122 of the planar light guide plate 104. LEDs 108 are disposed at both ends of the linear light guide plate 106. The linear light guide plate 106 is used for trimming the outgoing direction of light from the LEDs 108 which are point sources of light to establish a linear source of light. The linear light guide plate 106 has a plurality of notch-like recessed portions 142 formed in the surface facing the light incidence surface 122. Upon adjusting the density of formation of recessed portions 142, light that is uniformly distributed is emitted into the planar light guide plate 104 from the linear light guide plate 106.
Light emitted from the linear light guide plate 106 travels through the planar light guide plate 104 while being totally reflected by the mildly inclined surfaces 112 and by the light outgoing surface 128, whereby light incident on the steeply inclined surfaces 110 is emitted toward the reflection-type liquid crystal display panel 102. Light reflected by a reflection electrode formed on the pixel of the reflection-type liquid crystal display panel 102, passes through the planar light guide plate 104 and is emitted to the side of the display screen.
However, the conventional front-light reflection type liquid crystal display device using LEDs 108 as sources of light, involves the following two problems.
(1) Low Efficiency and Low Brightness.
Light emitted from the LEDs 108 falls on the planar light guide plate 104 through the linear light guide plate 106. Here, the utilization efficiency of light is a product of a utilization efficiency of light falling on the linear light guide plate 106 from the LEDs 108, a utilization efficiency of light in the linear light guide plate 106, and a utilization efficiency of light falling on the planar light guide plate 104 from the linear light guide plate 106. Therefore, the utilization efficiency of light inevitably decreases. In particular, when the point source of light is turned into a linear source of light by using a linear light guide plate 106, there exists a trade-off relationship between the light utilization efficiency and the uniformity in the distribution of light quantity, making it difficult to improve the light utilization efficiency. To accomplish a highly bright illumination, therefore, it is necessary to increase the number of the LEDs 108 since there is a limitation on the amount of light per each LED 108. FIG. 23 is a view illustrating the constitution of the front-light reflection type liquid crystal display device in which the number of the LEDs 108 is increased to four, as viewed from the side of the display screen. Referring to FIG. 23, when the number of the LEDs 108 is increased to four, the size of the linear light guide plate 106′ must be increased, offsetting the merit of decreasing the size by the use of LEDs 108.
(2) High Cost of Production.
The number of parts increases as compared to the conventional constitution of using the cold cathode tube. Besides, it is necessary to precisely arrange the linear light guide plate 106, planar light guide plate 104 and LEDs 108, resulting in an increase in the cost of production.
In order to solve the above two problems, there has been proposed a light guide plate of a structure in which light travels reciprocally, by providing a reflector on a surface (hereinafter referred to as light reflection surface) facing the light incidence surface 122 to reflect the incident light, and the reflected light is further reflected by the steep surfaces so as to be emitted to the side of the liquid crystal display panel 102. FIG. 24 is a view illustrating a front-light reflection type liquid crystal display device of the structure in which light travels reciprocally. FIG. 24A is a view of the structure of the reflection type liquid crystal display device of when it is viewed from the side of the display screen, and FIG. 24B is a sectional view of the reflection type liquid crystal display device cut along the line B—B in FIG. 24A. As shown in FIGS. 24A and 24B, an LED 108 is arranged at the center of the light incidence surface 122 of the light guide plate 120. A reflector 124 is provided on the surface of the light reflection surface 126 opposing the light incidence surface 122. A nearly flat light outgoing surface 128 is formed on the light guide plate 120 on the side of the liquid crystal display panel 102. Mildly inclined surfaces 113 and steeply inclined surfaces 111 are formed on the light guide plate 120 on the side facing the light outgoing surface 128. The surfaces 113 are mildly inclined toward the light reflection surface 126 at an angle of about 2° with respect to the light outgoing surface 128, and the surfaces 111 are steeply inclined toward the light incidence surface 122 at an angle of about 45° with respect to the light outgoing surface 128.
Light emitted from the LED 108 and is falling on the light guide plate 120, propagates through the light guide plate 120 while being totally reflected repetitively by the light outgoing surface 128 and by the mildly inclined surfaces 113, and is reflected by the reflector 124. The LED 108 is a point source of light. Therefore, light emitted therefrom travels toward the reflector 124 while spreading. Therefore, the distribution of light becomes nearly uniform on the reflector 124. Light is reflected by the reflector 124, falls again on the light guide plate 120, and is reflected by the steeply inclined surfaces 111 and is emitted toward the reflection type liquid crystal display panel 102.
Even by using the light guide plate 120 of the above structure in which light travels reciprocally, however, there remain problems as described below. Namely, light incident on the light guide plate 120 from the LED 108 mostly travels while being totally reflected by the mildly inclined surfaces 113 and the light outgoing surface 128. As described above, however, there exists an inclined angle of about 2° between the mildly inclined surfaces 113 and the light outgoing surface 128. Therefore, the angles of incidence of light incident on the surfaces 113 and 128 gradually decrease and finally become smaller than the critical angle. As shown in FIG. 24B, the ray 202 of light directly goes out of the light guide plate 120 through the mildly inclined surfaces 113 and the light outgoing surface 128 bringing about such a problem that the light utilization efficiency further decreases. In particular, the ray 202 of light that is directly emitted toward the display screen from the mildly inclined surfaces 113, is further reflected by the steeply inclined surfaces 111 and is emitted in a direction nearly perpendicular to the display screen, arousing a problem of a decrease in the contrast.
Though the distribution of light quantities becomes even on the reflector 124, the light distribution characteristics are not even since the angle of incidence of light to the reflector 124 differs depending upon the distance from the center of the reflector 124. Therefore, light is unevenly distributed as it is reflected by the reflector 124 and is guided again through the light guide plate 120 entering from the light reflection surface 126. Accordingly, light emitted into the reflection type liquid crystal display panel 102 from the light outgoing surface 128, is unevenly distributed, whereby brightness becomes irregular and the quality of display drops. This problem can be relaxed by rendering the surface of the reflector 124 to be scatter-reflection surface and by reflecting the ray 204 of light in a scattering manner, which, however, is not a complete solution. Besides, the light utilization efficiency inevitably drops due to the scattering of light or due to loss caused by scattering, and a complete solution is not obtained.
Moreover, the conventional front-light liquid crystal display device has a problem in that the contrast ratio of when the front-light is turned on is very smaller than the contrast ratio of when the back-light of the back-light liquid crystal display device is turned on. This is because, the light guide plate 120 has been disposed on the display screen side of the liquid crystal display panel 102, and a part of light irradiated from the front-light unit is reflected by the light outgoing surface 128 and the like of the light guide plate 120 instead of being reflected by the reflection electrode of the liquid crystal display panel 102.
To prevent the reflection by the light outgoing surface 128 and the like, in general, a reflection-preventing film is formed on the light outgoing surface 128 of the light guide plate 120 and on the surface of the liquid crystal display panel 102 on the side of the light guide plate 120. However, the polarizing plate 140 stuck to the surface of the liquid crystal display panel 102 and the light guide plate 120 are both formed of a resin, and it is not allowed to elevate the temperature of the substrate at the time of forming the film. Accordingly, a thin film of a high quality is not formed, the reflection is not prevented to a sufficient degree by the reflection-preventing film, and there remains a reflection factor of from 0.1 to 0.2%. Since the contrast ratio that is obtained is only about 10, it has been desired to further enhance the contrast ratio.